Sometimes Astrography is mistaken for the related science of Astrogation.

Overview:

Astrography

Astrography is the art and science of mapping out interstellar space. An astrographer is someone who maps out interstellar space.

Astrogation

Astrogation is the actual planning of the navigation of spacecraft. Or the planning of a route through space, either in interplanetary or interstellar travel. An astrogator is someone who navigates a spacecraft, or starship, through interstellar space.

Basic to any science of mapping is a coordinate system. The scientific system used by the Imperium is based on rings of longitude, rays of latitude, and parsecs. By convention, rays of latitude and rings of longitude are measured from a starting point at Reference (Core 0140), the world where the data from the First and Second Imperial Grand Surveys are stored.

The concentric ring passing through Reference/Capital is labeled the 10,000th ring, and is used as a baseline. Similarly, the ray of latitude extending from the center of the Galaxy through Reference/Capital is designated as the first ray. Measurement is in parsecs, counting in the trailing direction. Computation reveals that the circumference of the 10,000th ring (r=10,000) is 62,832 parsecs. Counting spinward uses a subtraction from 62,833, which is the equivalent of the zero baseline.

The format for expression of location is xxxx ring/ray yyyyy, where xxxx is the ring of longitude (distance from the galactic core in parsecs), and yyyyy is the distance of the ray of latitude (in parsecs) from the first ray of latitude, measured along the ring of longitude in the trailing direction.

Imperial Science: This mapping system is highly Imperio-centric, and other systems are used by other peoples and races outside the Imperium. This system has gained wide acceptance, however, among Imperium dominated client-states, human and otherwise. This mapping system breaks down and is prone to error beyond certain limits. It does serve admirably for a band approximately 400 parsecs wide at a longitude of 10,000 parsecs, which is a region precisely centered on the Imperium.

In ordinary discourse, a world is referred to by its name then sector and parsec/hex in parentheses. For example, the homeworld of Archduke Norris is Regina (Spinward Marches 1910): That is, the world Regina is in hex 1910 in Spinward Marches Sector.

When specifying the location of sectors, a coordinate system is used with Core sector being at (0, 0).

Thus the Spinward Marches would be at (-4, 1) and the Reavers' Deep would be at (-2, -2).